This invention relates to a method and apparatus for electrical measurement of oil quality, applicable to diesel fuel, or hydraulic, gearbox, transformer or engine oil, and preferably lubricating oil; and especially a method and apparatus for use in an engine, machine or filter.
It has been known for many years that the complex permittivity (or dielectric coefficient) of an engine lubricating oil changes with use, that is to say, both the real pan and the imaginary part change in response to changes in dissolved and suspended components of the oil. Such components are, for example, soot particles, water, acid combustion products, glycols, and ferrous and non ferrous metallic particles . In addition, oils often contain additives such as viscosity improvers and anti-oxidants which tend to break down with continued engine use, especially in the presence of water and metallic particles, which also accelerate the process of oxidation and general degradation.
It is also known that the reliability and longevity of an engine is crucially dependent upon the quality of its lubricating oil, and that an apparatus designed to detect some point at which the quality is deemed to be unacceptable would be desirable. In particular, when used in conjunction with a secondary bypass engine filter designed to remove particulate material down to 1 micron, such a device would be useful. Since such a filter may pass say 1% of the output from the oil pump, the differential pressure across the input and output of the filter is very low. making it difficult to measure. Consequently it is also difficult to know whether the filter element has become blocked. If this were to happen, however, the effect of the bypass filter in removing debris would be lost, and the concentration of contaminants would rise rapidly. Such a rise, or rate of change, could be detected by an oil quality monitor, enabling the filter or filter element to be replaced. If the oil quality monitor were to indicate poor quality, a sample may then be taken from the engine or machine, and sent to a suitable laboratory or facility for advanced spectrographic or chemical analysis, which may then reveal the presence of excessive soot, water, glycol, oxidation products, or metallic particles.
Also, degradation of most oils, such as due to oxidation or electrical breakdown, tends to result in the generation of products whose molecules are generally more polar than the oil from which they came. The base oil often comprises large hydrocarbon molecules which are generally only weakly polar, so the presence of most contaminants will result in an increase in one or both parts of the oil""s complex permittivity. An oil quality monitor which measures permittivity is therefore suitable for measuring changes in, for example, transformer oil, or the oil in gearboxes and transmissions which may be subjected to the high temperatures and agitation which are conducive to oxidation.
Transmission units and other hydraulic systems such as rams may also become contaminated with water as a result of working in wet or damp environments. Water will cause an increase in both parts of the oil""s complex permittivity, and may therefore be detected by apparatus according to the invention.
Advanced oil analysis as carried out by an accredited laboratory subjects oil samples to a battery of tests, one of which is often a basic measurement of dielectric properties, often carried out by hand. An oil quality monitor according to the invention may be incorporated into an automated production line. The invention may be adapted so that a sensing head is provided at the end of a slender flexible rod suitable for insertion into the dipstick orifice of, an engine or machine to allow in situ measurement of oil quality in an engine or machine not otherwise fitted with an oil quality measuring device.
In U.S. Pat. No. 3,182,255, Hopkins et al. describe a device in which a bridge circuit is used to measure the AC impedance of one arm of the bridge, which contains a capacitive element whose capacitance changes with the dielectric strength (sic) of a drop of lubricating oil.
This device requires the physical removal of an oil sample from the vehicle, and makes no distinction as to whether the measured parameter is the real or the imaginary part of the permittivity. It can be shown that the impedance of a capacitor containing a dielectric depends, to first order, upon the real part, and to second order, upon the imaginary part, so that, even if no change occurs in the real part, there will nevertheless be a change in the modulus, or magnitude, of the permittivity, if there is a change in the lossiness of the dielectric, such as may occur through the presence of carbon particles. It is this magnitude which is usually referred to loosely as xe2x80x9cdielectric constantxe2x80x9d. In another patent, U.S. Pat. No. 4,064,455, Hopkins et al. describe the use of an identical bridge circuit, but this time in conjunction with data storage and computational facilities.
In EP 0291363, Warenghem et al. describe a parallel plate capacitor, the capacitance of which varies with the concentration of carbon particles. In this document, capacitance is not defined as complex and is taken to refer to the modulus, or magnitude. No indication is given as to the means by which capacitance is actually measured.
In U.S. Pat. No. 4,733,556, Meitzler et al. describe a parallel plate capacitive sensor designed to fit between the engine block and the filter, where changes in the magnitude of capacitance are used to generate changes in the frequency of an associated oscillator. It is this change in frequency which is measured and subsequently compared, not with soot content of the oil, but with its viscosity. Although it is known that the complex permittivity of polar liquids will change with viscosity, it is also known that the viscosity will tend to increase with increasing soot concentration. This increase in viscosity is a macroscopic effect in the sense that a soot particle is many orders of magnitude larger than a molecule, and it is felt that measurement of dielectric constant is not a reliable indicator of viscosity in sooty oils.
In U.S. Pat. No. 4,345,202, Nagy et al. describe the use of microwaves in the range 8 to 12 GHz to measure soot content in engine oil, whereby nulls in the standing waves on a coaxial transmission line are located by means of a plurality of detectors arranged along the transmission line which is itself immersed in the sump of the engine
In U.S. Pat. No. 5,134,381, Schmitz et al. describe the use of a concentric capacitive sensor along whose axis passes, a fuel/alcohol mixture. Measurement of the capacitance of the sensor then provides the means whereby the alcohol content of the fuel may be determined, given a priori knowledge of the water content also. It appears that the capacitive sensor is excited by an external oscillator with the intention of measuring the impedance of the sensor and possibly also an associated phase shift.
Many other known devices are concerned with the measurement of the relative permittivity of fluids or fluid-like materials other than engine oil, many of which are distinctly multiphase. For example, in U.S. Pat. No. 2,121,920 a means for measuring the mass and moisture content of tobacco is described in which a parallel plate capacitive test cell is placed in series with an inductance. The resulting RLC circuit is tested for resonance by manually sweeping it with an oscillator, and measuring the magnitude and frequency of the response.
In U.S. Pat. No. 5,272,444, Cox describes a method for measuring the water content and salinity (water cut) of a petroleum stream via measurement of temperature, resistivity and dielectric constant, but gives no details as to the mechanical arrangement of the sensor. It is clear however that the sensor is excited by an oscillator running at one of two fixed frequencies viz. 15 MHz and 30 MHz.
In U.S. Pat. No. 4,932,243, Suh et al. describe an online means for measuring the moisture content of a material, for example polymer pellets, using a capacitive sensor comprising three conductive concentric cylinders through which the material passes axially. The capacitance and dielectric loss are determined using xe2x80x9cwell known techniquesxe2x80x9d which are not detailed.
In U.S. Pat. No. 4,288,741, Dechene et a). describe a method for measuring both displacement and conduction currents in a two phase fluid of which one is assumed to be conductive, to determine the relative fractions of the phase. This is achieved by driving a capacitive sensor via an oscillator of fixed frequency with the intention of separating the relatively large conduction current from the relatively small capacitive current due to the AC excitation voltage.
In U.S. Pat. No. 4,181,881, Preikschat describes an apparatus also intended to measure the so-called dielectric coefficient and conductivity of various materials. This appears to be a batch testing method, and precise details of the capacitive sensor are not provided, other than to describe it as an earthed rectangular box with an active centre electrode. The capacitive sensor is excited by means of a stable crystal oscillator, the bridge circuitry being designed to measure the phase and amplitude of the voltage across the sensor.
In U.S. Pat. No. 3,979,581, Reulend describes a method of measuring the mass of tobacco by exciting a capacitive sensor with a signal from a swept external oscillator. In this arrangement a discriminator measures the frequency while a demodulator and differentiator locate the frequency and amplitude at which amplitude resonance occurs. Precise details of the capacitive sensor are not provided, but it appears to be a parallel plate capacitor of indeterminate size, possibly of the same order of magnitude as a cigarette.
In GB 2249636, McBrearty describes an inline dielectric sensor using a form of interdigitating capacitor to measure the dielectric coefficient and loss factor of molten polymers. This is accomplished by exciting the sensor with a sine wave generator. In the document, it is stated that a current to voltage converter and a lock-in amplifier are used to measure the amplitude and phase of the resultant alternating current.
WO 96/28742 describes an apparatus intended specifically for on-line monitoring of engine lubricating oils in diesel engines, and discusses the necessity or desirability of carrying out continuous monitoring of the oil. In this apparatus, the fundamental principal is that of measuring the dielectric coefficient of the oil, but in order to extract the greatest amount of information from the sample, the apparatus uses an arrangement of electromagnets to concentrate ferrous particles in the vicinity of a flat interdigitating capacitive sensor. Although the interdigitating capacitor is an interesting and useful configuration, the given formula relating capacitance to the dimensions of the capacitor refers to a parallel plate configuration, and so appears to be inaccurate in that context. The capacitor forms part of an oscillator circuit whose frequency varies with the dielectric coefficient of the oil. Since no inductor is present in the block diagram or in the text, it is assumed that the circuit is found to be self resonant as a result of parasitic inductances or the self inductance of the capacitive sensor. The inventors state that xe2x80x9cthe sensor element is charged by an oscillator circuit . . . using a monostable multivibrator to generate an output signal at a frequency corresponding to the sensor element capacitancexe2x80x9d. This can be taken to mean that the oscillator frequency is determined directly by the sensor capacitance, or that the frequency of the oscillator is adjusted until it coincides with the self resonant frequency of the sensor. In any event, the measurand is the frequency, so the parameter actually being measured is the magnitude of the dielectric coefficient (sometimes referred to as xe2x80x9cdielectric constantxe2x80x9d).
It is known that a few devices have attempted to correlate oil quality with dielectric coefficient by measuring the capacitance of a capacitor with the oil as a dielectric. This has been achieved by measuring either the change in AC impedance or by measuring the change in frequency when connected in an LC resonant circuit. However, in these cases what is actually being measured is the magnitude of the capacitance, which changes slightly with the lossiness Tan xcex4 of the dielectric, but also changes with dielectric constant. Such devices therefore have the disadvantages of being sensitive to oil base type, and only being sensitive to the second order as regards the loss term Tan xcex4.
The present invention provides apparatus for measuring oil quality based on the permittivity (dielectric coefficient) of the oil comprising a capacitive sensor for exposure to the oil, and an oscillator circuit including the sensor, characterised in that the oscillator circuit comprises an LC or crystal oscillator and provides an output signal, the amplitude of which is dependent upon the lossiness Tan xcex4 of the oil, and measuring means that responds to the amplitude of said output signal as a measure of oil quality.
In particular, the amplitude can be inversely proportional to the Tan xcex4 value. Tan xcex4 is the ratio e xcex5xe2x80x3/xcex5xe2x80x2 where xcex5xe2x80x3 is the imaginary part of the complex relative permittivity and xcex5xe2x80x2 is the real part. The oil can be diesel fuel or another light mineral oil. The oil is preferably lubricating oil.
Thus, the output of the oscillator varies in response to changes in the lossiness of the dielectric medium (the oil), which in turn are determined principally by changes in the oil""s soot content, acidity, and polar oxidation products. It is this change in the amplitude of the oscillator output which provides a measure of the oil quality.
The present invention is applicable to diesel fuel and oil in engines, or oil in hydraulic transmissions or machines, or oil in electrical machines where contaminants may be introduced into the oil by electrical breakdown or ingress of moisture.
The present invention also relates to an engine or machine including said electrical measuring apparatus. In particular, the sensor may be fitted to the oil supply tunnel or sump of the engine or machine. Alternatively, the sensor may be fitted to an oil filter, such as an oil bypass filter, attached to the engine.
The present invention also relates to an oil bypass filter for attachment to an engine, the oil bypass filter including said electrical measurement apparatus.
The present invention also relates to a method of measuring oil quality based on the permittivity of the oil using an oscillator circuit including a capacitive sensor, characterised in that the sensor is an LC or crystal oscillator which provides an output signal, the amplitude of which is dependent upon the lossiness Tan xcex4 of the oil, and is used to give a measure of oil quality.
The sensor is preferably incorporated in a sensor head which is generally concentric or radially symmetric. It is preferably perforated or slotted to allow the free passage of oil over the electrically active surfaces of the sensor. Typically, the sensor head is about 10 mm in diameter, being small enough to fit into a hole such as might be provided in an engine for an oil pressure switch, such as a xe2x85x9xe2x80x3 NPT (National Pipe Thread) or xc2xcxe2x80x3 BSP (British Standard Pipe) orifice. The associated oscillator electronics are located in the space behind the sensor head, which can take the form of a hollow hexagonal nut approximately 30 mm A/F by 20 mm deep. Since the electronic circuitry is at approximately the same temperature as the oil, it comprises components which will operate in elevated temperatures, for example, up to 150xc2x0 C., and, optionally, which will also provide compensation to allow for temperature dependent changes in the dielectric medium.
It is known that changes in temperature affect electronic components such as transistors, inductors and capacitors, so that the output of the circuit, independently of the oil dielectric, is a function of temperature. The electrical properties of the oil itself are also affected by temperature. The most visible effect is the change in density, which is effectively linear over the range of interest, that is, from about 30xc2x0 C. to 150xc2x0 C. It follows that the concentration of contaminants, being inversely proportional to volume, will fall with increasing temperature. Simultaneously, however, the viscosity of the oil also falls with temperature, allowing greater freedom of motion for the constituent molecules, and it can be shown theoretically that the greater the average dipole moment of the liquid, the greater will be its dependence on temperature. In order to operate accurately over a wide range of temperatures, some form of compensation is necessary. Incorporated into the electronics is a small temperature sensor whose output, after suitable buffering, is passed out of the sensor head to a display unit.
The display unit incorporates a microprocessor which accepts as input the output voltage from the oscillator and the output from the temperature sensor. By means of a suitable lookup table or algorithm, this allows the appropriate adjustments to be made in order to render the final indication independent of temperature over the range say 30xc2x0 C. to 150xc2x0 C.
Generally, the curve showing oscillator output versus temperature is a bivariate function of permittivity and temperature, so full compensation requires a two dimensional lookup table or algorithm. However, it is found in practice that with the correct choice of operating frequency the slope of the curve is only weakly dependent on the concentration of contaminants, allowing the use of a simpler one dimensional table. FIG. 9 shows a number of typical curves where the change in oscillator output is plotted against temperature. The upper curve is for a clean engine oil, while the lower curves are for progressively more contaminated engine oil.
Oils are generally a complex mixture of hydrocarbons and proprietary additives, such that the output of the oscillator differs slightly according to the precise formulation. Although it has been found in practice, that the output differs by only a few percent to within one standard deviation over a wide range of different oils from different manufacturers (as shown in FIG. 10), calibration means is included to eliminate even this small source of error. When the vehicle or machine is serviced or supplied with fresh oil, the microprocessor may accept an input from a recessed switch or coded transmitter when the correct operating temperature has been reached, such that that particular value is held in memory, and therefore becomes the baseline against which all subsequent readings will be compared.
In the event of the main power supply being cut, such as removal of the vehicle or machine""s battery, the contents of the memory are retained in, for example, an electrically erasable programmable read only memory (EEPROM). This same EEPROM may also hold several hundred sets of readings sampled over the lifetime of the oil, typically 300 to 500 hours. The data may then be downloaded into a computer for subsequent examination and analysis, or it may be suitably encoded and transmitted to a remote location by radio for example. In addition to the measured value of the oil""s quality, the rate of change of the device""s output is an important indicator of engine health. If the slope of output versus time should unexpectedly increase, this would tend to indicate a sudden increase in contaminants, requiring immediate remedial attention before damage should occur. In order to improve the reliability of the data, the microprocessor accepts for example 10 measurements at short intervals, such that the final value displayed or stored is the arithmetic mean of the measured values.
The final, visible part of the microprocessor""s output may take the form of a warning light or buzzer, or for test and diagnostic purposes may take the form of an alphanumeric display.
In the preferred sensor, the body of the sensor is at ground potential, while an integral automotive style connector provides one pin for the output signal, one pin for the temperature output, and a further pin for DC power input, which may be 12V or 24V.
In one embodiment, the apparatus is used in conjunction with a secondary filter. In this way, oil drain intervals may be extended with consequent cost savings, and the reliability of the engine is enhanced by the use of oil of consistently high quality. Additionally, analysis of the oil can provide an indication of faults in the engine allowing early rectification and the planned scheduling of preventive maintenance to avoid expensive downtime.
The preferred apparatus according to the present invention is, advantageously, of very low cost, for example, comparable with that of an inexpensive automotive pressure sensor, and of small size, and requiring little or no modification to the engine or machine. It is preferably intended to be fitted permanently to the engine or secondary filter, and to monitor the quality of the oil. continuously throughout the life of the engine or machine. It is preferably insensitive to changes in temperature, within a range of, say, 30xc2x0 C. to 150xc2x0 C., or in the case of hydraulic oils, from 20xc2x0 C. to 100xc2x0 C. It is insensitive to changes in base oil type from one manufacturer to another. In a preferred embodiment, the apparatus is powered directly by the engine or vehicle 12V or 24V electrical supply, and provides a single output consisting of an analogue voltage which varies in response to changes in the electrical properties of the oil. This signal may be used to drive a dashboard display which indicates the current state of the oil to the operator, and which may include a visible or, audible warning to indicate a filter change, oil change, or sample to be sent for analysis.
In a further embodiment, the output may be passed to the vehicle""s on-board computer, where the data may be sampled periodically and stored in memory for real time analysis or for subsequent downloading and examination.
In a further embodiment the sensor comprises an oscillator running at, say, 200 to 300 MHz and mounted on a printed circuit board (PCB) approximately 8 mm wide by 20 mm long. This small size is achieved through the use of surface mount components. The active sensing head is attached directly to the PCB and surrounded by a perforated metallic cover which provides mechanical protection, electrical screening and a return path for radiated electromagnetic energy. A small temperature sensitive element such as a thermistor is also fitted, and its output carried along a flexible hollow tube such as the outer part of a Bowden cable. The oscillator is supplied with power from a separate regulated supply, which is also fed down the hollow tube. Careful choice of the bias point of the oscillator will ensure that, as the oscillator voltage varies with Tan xcex4, so also does the current, which then reflects any change in Tan xcex4 due to contamination of the oil. Temperature compensation is applied in the manner already referred to by measuring the output from the temperature sensor, and then applying a correction in the display unit. In a further embodiment, the output from the temperature sensor is digitised and carried on the power supply lead. An active sensor of this size may advantageously be used for attachment to the end of a vehicle""s dipstick (normally used for checking oil levels), or it may form part of a separate instrument for use by garages and testing stations to carry out a quick check on oil quality for stationary vehicles or machines. The sensor may be used for measuring the quality of oil in an engine, hydraulic machine, gearbox, or any other apparatus using non polar mineral oils.